SELECTED PAPERS

“Photophysics and photochemistry of pterins in aqueous solution”

C. Lorente, A. H. Thomas, Acc. Chem. Res., 39, 395-402 (2006).
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Abstract: Pterins belong to a family of heterocyclic compounds present in a wide range of living systems and participate in relevant biological functions. Interest in the photochemistry and photophysics of this group of compounds has increased since the participation of pterin derivatives in different photobiological processes has been suggested or demonstrated in recent decades. This account describes and connects basic studies on the fluorescence emission, the photooxidation, and the photosensitizing properties of oxidized 6-substituted pterins in aqueous solution under UV-A irradiation. The biological implications of these studies are also discussed.


“Oxidation of 2'-deoxyguanosine-5'-monophosphate photoinduced by pterin: Type I versus Type II mechanism”

G. Petroselli, M. L. Dántola, F. M. Cabrerizo, A. L. Capparelli, C. Lorente, E. Oliveros, A. H. Thomas. J. Am. Chem. Society, 130, 3001-3011 (2008).
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“The photosensitizing activity of lumazine using 2´-deoxyguanosine 5´-monophosphate and HeLa cells as targets”

M. P. Denofrio, S. Hatz, C. Lorente, F. M. Cabrerizo, P. R. Ogilby, A. H. Thomas, Photochem. Photobiol. Sci., 8, 1539–1549 (2009).
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“Photochemistry of dihydrobiopterin in aqueous solution”

M. Vignoni, F. M. Cabrerizo, C. Lorente, C. Claparols, E. Oliveros, A. H. Thomas, Org. Biomol. Chem., 8, 800–810 (2010).
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Photooxidation of pterins takes place in vivo under pathological conditions. We have investigated the photochemistry of dihydrobiopterin and discuss the mechanisms involved.


“Electron transfer processes induced by the triplet state of pterins in aqueous solutions”

M. L. Dántola, M. Vignoni, C. González, C. Lorente, P. Vicendo, E. Oliveros, A. H. Thomas, Free Rad. Biol. Med., 49, 1014–1022 (2010).
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Pterins (Pt) are heterocyclic compounds widespread in living systems. They participate in relevant biological processes, such as metabolic redox reactions, and can photoinduce the oxidation of biomolecules through electron-transfer mechanisms. We have investigated the electron-transfer pathways initiated by excited states of pterin (Ptr) and 6-methylpterin (Mep), selected as model compounds. The experiments were carried out in aqueous solutions under continuous UV-A irradiation, in the presence and in the absence of ethylenediaminetetraacetic acid (EDTA), used as an electron donor. The reactions were followed by UV/Vis spectrophotometry, HPLC, and an enzymatic method for H2O2 determination. The formation of the superoxide anion (O2•−) was investigated by electron paramagnetic resonance–spin trapping. The triplet excited states of Ptr and Mep are efficient electron acceptors, able to oxidize a Pt molecule in its ground state. The resulting radical anion (Pt•−) reacts with dissolved O2 to yield O2•−, regenerating the pterin. In the presence of EDTA, this reaction competes efficiently with the anaerobic reaction between Pt•− and EDTA•+, yielding the corresponding stable dihydroderivatives H2Pt. The effects of EDTA and dissolved O2 concentrations on the efficiencies of the different competing pathways were analyzed.



“Mechanism of photooxidation of folic acid sensitized by unconjugated pterins”

M. L. Dántola, M. P. Denofrio, B. Zurbano, C. S. Gimenez, P. R. Ogilby, C. Lorente, A. H. Thomas, Photochem. Photobiol. Sci., 9, 1604–1612 (2010).
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When folic acid is exposed to UV-A radiation in an air-equilibrated aqueous solution, the rate of its degradation increases with the time due to a photosensitized-electron transfer process.



“Production and quenching of reactive oxygen species by pterin derivatives, an intriguing class of biomolecules”

E. Oliveros, M. L. Dántola, M. Vignoni, A. H. Thomas, C. Lorente, Pure App. Chem., 83, 801-811 (2011).
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Pterins, a family of heterocyclic compounds derived from 2-aminopteridin-4(1H)-one, are widespread in living systems and participate in important biological functions, such as metabolic redox processes. Under UV-A excitation (320–400 nm), aromatic pterins (Pt) can generate reactive oxygen species (ROS), as a consequence of both energy- and electrontransfer processes from their triplet excited state. Quantum yields of singlet oxygen (1O2) production depend largely on the nature of the substituents on the pterin moiety and on the pH. Formation of the superoxide anion by electron transfer between the pterin radical anion and molecular oxygen leads to the production of significant amounts of hydrogen peroxide (H2O2) by disproportionation. Dihydropterins (H2Pt) do not produce 1O2 but are oxidized by this species with high rate constants yielding pterins as well as H2O2. In contrast to aromatic derivatives, H2Pt are oxidized by H2O2, and rates and products strongly depend on the nature of the substituents on the H2Pt moiety. Aromatic pterins have been found in vivo under pathological conditions, e.g., biopterin or 6-carboxypterin are present in the skin of patients affected by vitiligo, a depigmentation disorder. The biomedical implications of the production of ROS by pterin derivatives and their reactivity with these species are discussed.



“Emission properties of dihydropterins in aqueous solutions”

M. P. Serrano, M. Vignoni, M. L. Dántola, E. Oliveros, C. Lorente, A. H. Thomas, Phys. Chem. Chem. Phys., 13, 7419–7425 (2011).
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Pterins are involved in photobiological processes and some of them are present in the skin of humans under pathological conditions.




“Inactivation of tyrosinase photoinduced by pterin”

M. L. Dántola, A. D. Gojanovich, A. H. Thomas, Biochem. Biophys. Res. Commun., 424, 568–572 (2012).
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Tyrosinase catalyzes in mammals the first and rate-limiting step in the biosynthesis of the melanin, the main pigment of the skin. Pterins, heterocyclic compounds able to photoinduce oxidation of DNA and its components, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder in which the protection against UV radiation fails due to the lack of melanin. Aqueous solutions of tyrosinase were exposed to UV-A irradiation (350 nm) in the presence of pterin, the parent compound of oxidized pterins, under different experimental conditions. The enzyme activity in the irradiated solutions was determined by spectrophotometry and HPLC. In this work, we present data that demonstrate unequivocally that the enzyme is photoinactivated by pterin. The mechanism of the photosensitized process involves an electron transfer from tyrosinase to the triplet excited state of pterin, formed after UV-A excitation of pterin. The biological implications of the results are discussed.



“Photosensitizing properties of biopterin and its photoproducts using 2’-deoxyguanosine 5’-monophosphate as an oxidizable target”

M. P. Serrano, C. Lorente, F. E. Morán Vieyra, C. D. Borsarelli, A. H. Thomas. Phys. Chem. Chem. Phys., 14, 11657–11665 (2012).
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Pterins (PT), present in the skin under pathological conditions, photoinduce the oxidation of 2’-deoxyguanosine 5’-monophosphate (dGMP) through an electron-transfer-initiated process.



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